1. Field of the Invention
The present invention relates to a crystal unit having a stacked structure in which a plurality of crystal plates are joined to each other by direct bonding, and particularly to a configuration for extending an electrode to an outer surface of a crystal unit.
2. Description of the Related Art
A crystal unit having a configuration in which a vibration member formed by a quartz crystal plate is hermetically encapsulated in a package is known as a frequency control device, and is incorporated in an oscillation circuit and a tuning circuit in various electronic apparatuses. As electronic apparatuses have recently been miniaturized, the crystal unit also is required to be miniaturized and simplified in configuration, and accordingly, there has been proposed a crystal unit having a stacked structure in which crystal plates are joined to each other by direct bonding using, for example, siloxane bond. A crystal unit having such a stacked structure is also called a stacked crystal unit. Such a stacked crystal unit is disclosed, for example, in Japanese Patent Laid-Open Application Nos. 8-204479 (JP-A-8-204479), Japanese Patent Laid-Open Application No. 2000-269775 (JP-A-2000-269775), and Japanese Patent Laid-Open Application No. 2001-119263 (JP-A-2001-119263).
FIG. 1A is a plan view of a conventional stacked crystal unit, and FIG. 1B a cross-section view taken along line A-A in FIG. 1A.
Concerning quartz crystal, three crystallographic axes X, Y and Z are generally defined based on crystallography. Description will be hereinafter provided using the crystallographic axes defined crystallographically.
A crystal unit shown is configured in a manner that second and third quartz crystal plates 1b and 1c are directly bonded to both principal surfaces of first quartz crystal plate 1a so that orientations of individual crystallographic axes coincide with each other. First crystal plate 1a functions as a vibration member, and includes, for example, crystal blank 2 having a shape of a tuning-fork, and has a configuration that a bottom surface of tuning-fork base portion 2a is linked or connected to outer circumferential frame portion 3 through the intervention of protruding bar 3a. Here, it is assumed that the longitudinal direction of tuning-fork-like crystal blank 2 coincides with a Y axis, its width direction coincides with an X axis, and its thickness direction coincides with a Z axis. Such tuning-fork-like crystal blank 2 vibrates in piezoelectric vibration of tuning-fork vibration mode. On tuning-fork arms 2b of tuning-fork-like crystal blank 2, a pair of excitation electrodes (not shown) are formed, and first and second extending electrodes 4a and 4b connected to the excitation electrodes extend from one principal surface of first crystal plate 1a in tuning-fork base portion 2a to electrode pads 4x and 4y on both end portions through a surface of frame portion 3.
Second and third crystal plates 1b and 1c function as a cover for the vibration member, that is, a vibration region, of first crystal plate 1a, and both have a concave portion in an area opposite to the vibration member (i.e., tuning-fork-like crystal blank 2). Then, to both principal surfaces of frame portion 3 in first crystal plate 1a, open end surfaces to form outer circumferential portions of the concave portions in second and third crystal plates 1b and 1c are joined by direct bonding. Direct bonding is processing that mirror polished surfaces are hydrophilized, specifically, the surfaces are modified with a hydroxyl group (—OH group), and subsequently, the surfaces are made opposite to each other and heated while being pressed, thereby siloxane (Si—O—Si) bond is formed between both crystal plates, which bonds both crystal plates to each other.
Then, for example, on an outer circumstantial portion of both end portions of an outer surface of second crystal plate 1b, there is provided a pair of external terminals 5 used for surface-mounting the crystal unit on a wiring board. The pair of external terminals 5 is electrically connected to electrode pads 4x and 4y to which first and second extending electrodes 4a and 4b of first crystal plate 1a extend via electrode through-holes 6 provided in both end portions of second crystal plate 1b. Electrode through-hole 6 is formed by filling a through-hole provided in advance with material fitted for quartz, for example, conductive paste containing chromium (Cr), and subsequently burning or sintering. This electrically connects electrode pads 4x and 4y to external terminals 5, respectively, and hermetically seals the through-holes. That is, a via-hole is formed.
Such a crystal unit has three components, that is, first to third crystal plates 1a to 1c, so that its configuration is simplified, and the crystal unit can be formed in a small size. Further, the orientations of the three crystallographic axes X, Y and Z of first to third crystal plates 1a to 1c coincide with each other, and therefore the crystal plates have a similar thermal expansion coefficient, which provides better aging characteristics against change in temperature.
In the crystal unit having the configuration described above, electrode pads 4x and 4y of extending electrodes 4a and 4b extending to frame portion 3 of first crystal plate 1a are extended through the via-holes (i.e., electrode through-holes 6) of second crystal plate 1b, and connected to external terminals 5 on second crystal plate 1b. In this case, frame portion 3 of first crystal plate 1a is directly bonded to the outer circumference of second crystal plate 1b, but first and second extending electrodes 4a and 4b, and electrode pads 4x and 4y are basically bonded directly to neither first crystal plate 1a nor second crystal plate 1b. As the result, for example as shown in FIG. 2, on a bonded interface between the crystal plates, there are produced gaps around first and second electrode pads 4x and 4y due to an electrode film thickness of electrode pads 4x and 4y including first and second leading electrodes 4a and 4b, dependent on the corresponding electrode film thickness. Then, airtightness of the crystal unit is maintained basically by electrode through-holes 6 formed as via-holes.
However, it is thought that the via-hole serving as electrode through-hole 6 is formed of a sintered member of chromium, and the via-hole is not integrated with quartz crystal by interatomic bond, and does not necessarily provide reliable air sealing, dependent on manufacture conditions and the like. Then, electrode through-holes 6 are provided only in second crystal plate 1b, and as the crystal unit becomes increasingly lower in height, a sealing path length is shorter between filling material (e.g., chromium) and a side wall of the crystal plate at the position of the electrode through-hole. Accordingly, there has been a problem that it was difficult to secure airtightness, and the vibration member was susceptible to an effect of external environments.
In addition, in a crystal unit using a package made of ceramics other than quartz crystal, measures to improve airtightness of a via-hole are disclosed in Japanese Patent Laid-Open Application No. 2000-77942 (JP-A-2000-077942).